1. Field of the Invention
The present invention relates to a negative power supply control circuit which controls a negative voltage regulator circuit with a positive voltage control signal and a power supply device which has the negative power supply control circuit.
2. Description of Related Art
An imaging device such as a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) image sensor requires both of positive and negative voltages as a drive voltage. Thus, an electronic device which has the imaging device is provided with a positive voltage regulator circuit which outputs a positive voltage and a negative voltage regulator circuit which outputs a negative power supply voltage. For the positive voltage regulator circuit and the negative voltage regulator circuit, reset control of the circuit or on/off control of output is performed by a microcomputer or the like in some cases. In order to perform such processing to the negative voltage regulator circuit, it is necessary to convert a positive voltage control signal which is output from the microcomputer into a negative voltage control signal and output the converted signal to the negative voltage regulator circuit.
Japanese Patent Application Laid Open Publication No. 2012-063849 shows a negative power supply control circuit 100 (see FIG. 1) that converts a positive voltage control signal, which is input to a control signal input terminal Vcont0, into a negative voltage control signal Va0. In the negative power supply control circuit 100 shown in FIG. 1, when a control signal at a low level is input to the control signal input terminal Vcont0, a current I1 which flows in a voltage current conversion circuit 110 is zero. By a current source 120 drawing a current I2 from a connection point a, a control signal Va0 at a low level (negative voltage) is output to the connection point a. When a control signal (positive voltage) at a high level is input to the control signal input terminal Vcont0, the current I1 flows in the voltage current conversion circuit 110. The current I1 becomes larger than the current I2 of the current source 120, and raises the voltage at the connection point a. Furthermore, the clamp circuit 130 draws part of the current I1 to clamp the voltage of the connection point a to the ground potential. Thus, it is possible to output a control signal Va0 at a high level (which is nearly the ground potential) to the connection point a.
As described above, according to the negative power supply control circuit 100 in FIG. 1, it is possible to convert the positive voltage control signal, which is input to the control signal input terminal Vcont0, into the negative voltage control signal Va0. However, in the negative power supply control circuit 100 in FIG. 1, when the voltage value of the control signal input terminal Vcont0 becomes higher than a defined value, there arises a problem of large increase in the current I1 flowing in the voltage current conversion circuit 110. That is, the current drawn into the circuit from the control signal input terminal Vcont0 becomes greatly large. Normally, the control signal is output from the microcomputer to the control signal input terminal Vcont0. However, in general, the microcomputer does not have any output terminal capable of applying the large current. Thus, the increase in the current of the control signal input terminal Vcont0 causes a problem that the control signal which is input to the control signal input terminal Vcont0 cannot be maintained in a normal voltage range.
Furthermore, in the negative power supply control circuit 100 in FIG. 1, when the voltage value of the control signal input terminal Vcont0 is higher than the defined value, the current drawing by the clamp circuit 130 cannot catch up with the increase in the current I1 of the voltage current conversion circuit 110. When the current drawing cannot catch up with the increase in the current I1, there arises a problem that the voltage of the control signal Va0 becomes a voltage (positive voltage) higher than the ground potential.
In the negative power supply control circuit, if it is possible to handle a plurality of types of control signals having different voltage values as the input positive voltage control signal, it is possible to incorporate a same circuit into a plurality of types of devices having different specifications and obtain effects such as reduction of manufacturing cost of the circuit. However, in the negative power supply control circuit 100 in FIG. 1, there is a problem that it is difficult to successfully handle a case where the control signal of a high voltage is input to the control signal input terminal Vcont0 as described above.
An object of the present invention is to provide a negative power supply control circuit which can suppress the increase in current flowing into the circuit from the control signal input terminal and suppress the excess of the converted negative voltage control signal over the ground potential even when a control signal of a high voltage is input to the control signal input terminal. In addition, an object of the present invention is to provide a power supply device which has such a negative power supply control circuit.